In recent years, due to noise reduction in engines and the accompanying reduction of sound insulation equipment, the lowering of noise in automotive alternators mounted to the engines is in demand. Noise in automotive alternators can be classified roughly into two groups including wind noise generated by fans and electromagnetic noise, and higher harmonic electromagnetic noise is particularly a problem, being harsh in tone.
Generally, it is known that magnetic attraction force proportional to rotational frequency is generated in automotive alternators in air-gap portions between claw-shaped magnetic pole portions of a field rotor and teeth of a laminated core of a stator, electromagnetic noise being generated by component parts resonating at their excitation frequencies. It is difficult to avoid resonance in component parts in automotive alternators because the working rotational frequency range is spread over 1,000 to 20,000 revolutions per minute (rpm). In a stator, in which the resonating frequency is comparatively low, resonance occurs at 1,000 to 5,000 rpm, and in a field rotor, in which the resonating frequency is comparatively high, the claw-shaped magnetic pole portions resonate in the vicinity of 9,000 rpm. Because the noise level of this electromagnetic noise is affected by the magnitude of the vibrational amplitude, it is important to suppress vibrations to reduce electromagnetic noise.
Thus, for high-speed electromagnetic noise, attempts have been made to reduce electromagnetic noise by engaging a vibration-suppressing ring in an interior portion of the claw-shaped magnetic pole portions to shift the resonating frequency of the claw-shaped magnetic pole portions to a frequency band higher than the working rotational frequency range, or to reduce electromagnetic noise by applying a resin to the claw-shaped magnetic pole portions to dampen vibration of the claw-shaped magnetic pole portions. On the other hand, for low-speed electromagnetic noise, attempts have been made such as increasing rigidity of the laminated core or the holding case of the laminated core, suppressing eccentricities in and deformation of the laminated core, or applying a resin to dampen vibration of the laminated core. Furthermore, magnetic noise can be reduced by widening the gap between the laminated core and the field rotor to reduce magnetic attraction force, but this countermeasure leads to declines in output.
However, from the viewpoint of improving output, automotive alternators of this kind are generally designed such that the axial length of the laminated core of the stator is shorter than the axial length of a Lundell-type core, and the outer radius of cylindrical portions is smaller than the outer radius of the field rotor, structurally making for constructions which vibrate easily. Thus, since magnetic attraction force is concentrated at tip portions of the claw-shaped magnetic pole portions and the vibrational amplitude in the radial direction increases, and in addition, the distance from the cylindrical portions to the claw-shaped magnetic pole portions is great, vibrational amplitude in an axial direction resulting from magnetic attraction force is increased, preventing electromagnetic noise from being reduced effectively even if the countermeasures described above are applied.
An attempt is proposed in Japanese Patent Non-Examined Laid-Open No. HEI 11-243673, for example, to try to achieve high output by making the laminated core of the stator and the yoke portions of the field rotor face each other to minimize the magnetic flux leaking outside from the yoke portions and thereby increase the rate of recovery of the magnetic flux generated by the field rotor.
In Japanese Patent Non-Examined Laid-Open No. HEI 11-243673, as shown in FIG. 13, it is stated that high output can be achieved by setting a ratio (Lc/Lp) between an axial length Lc of a laminated core 51 of a stator and an axial length Lp of a Lundell-type core 52 of a field rotor to a range from 0.7 to 1.0 and setting a ratio (R2/R1) between an outer radius R1 of the Lundell-type core 52 and an outer radius R2 of cylindrical portions 52a to a range from 0.54 to 0.60.
However, in Japanese Patent Non-Examined Laid-Open No. HEI 11-243673, because the ratio (Lc/Lp) between the axial length Lc of the laminated core 51 and the axial length Lp of the Lundell-type core 52 is set to a range from 0.7 to 1.0, the surface area of root portions of claw-shaped magnetic pole portions 52c facing the laminated core 51 is large. Thus, because the fluctuations of the magnetic poles passing the laminated core 51 are smooth and change smoothly in an axially central portion of the laminated core 51, an alternating voltage close to a sine wave is generated. On the other hand, at axial end portions of the laminated core 51, the root portions of the claw-shaped magnetic pole portions 52c and the tapered tip portions of the claw-shaped magnetic pole portions 52c pass the laminated core 51 alternately. The pass time of the root portions of the claw-shaped magnetic pole portions 52c is long, whereas the pass time of the tip portions of the claw-shaped magnetic pole portions 52c is short, generating an alternating voltage having a disrupted sine wave. The generation of this alternating voltage having a disrupted sine wave acts to increase magnetic vibration of the laminated core 51, and one problem has been that low-speed electromagnetic noise is increased by the increased vibration of the stator. In addition, when Lc becomes long (as Lc/Lp approaches 1), at axial end portions of the laminated core 51, the fluctuations of the magnetic poles are abrupt because the tip portions of the claw-shaped magnetic pole portions 52c do not face the laminated core 51 and shoulder portions of the Lundell-type core 52 are not chamfered. Hence, an alternating voltage having an even more disturbed sine wave is generated, further increasing low-speed electromagnetic noise.
Because the ratio (R2/R1) between the outer radius R1 of the Lundell-type core 52 and the outer radius R2 of the cylindrical portions 52a is set to a range from 0.54 to 0.60, another problem has been that the contact surface area between the bobbin on which the field coil is wound and the yoke portions 52b of the Lundell-type core 52 decreases, lowering the damping effect suppressing vibration of the claw-shaped magnetic pole portions 52c, thereby worsening high-speed electromagnetic noise.
In addition, because Lc/Lp is set to a range from 0.7 to 1.0, the end surfaces of the laminated core 51 are positioned near the end surfaces of the Lundell-type core 52. Consequently, in the coil end groups of the armature coil, the root ends, which have the most irregularities in a circumferential direction, radially face the shoulder portions, where pressure fluctuations are greatest in the Lundell-type core 52. Thus, yet another problem has been that the root portions of the coil end groups and the shoulder portions of the Lundell-type core 52 interfere with each other due to rotation of the Lundell-type core 52, increasing wind noise.